1. Field of the Invention
The present invention relates to a magnetic recording medium having provided on a non-magnetic support a lower layer formed by dispersing a non-magnetic powder and/or a magnetic powder in a binder and having provided on the lower layer at least one magnetic layer formed by dispersing a ferromagnetic powder in a binder.
2. Description of the Related Art
Magnetic recording media are widely used as recording tapes, video tapes, flexible disks, etc. In the magnetic recording media, a magnetic layer in which a ferromagnetic powder is dispersed in a binder is layered on a non-magnetic support. The magnetic recording media are required to have high levels of various properties such as electromagnetic conversion characteristics, transport durability, and transport performance. That is, audio tapes for recording and playing back music are required to have high playback performance of original sounds, and video tapes are required to have excellent electromagnetic conversion characteristics such as playback performance of original images. At the same time as being required to have excellent electromagnetic conversion characteristics as described above, the magnetic recording media are required to have good transport durability.
However, since a magnetic layer obtained by coating a support with a coating solution in which a ferromagnetic fine powder has been dispersed in a binder has a high degree of packing of the ferromagnetic fine powder and a low value for elongation at break and is brittle, if the magnetic layer is formed without providing an undercoat layer, it might be easily destroyed by the application of mechanical force and peeled off from the support. An undercoat layer is therefore provided on the support so as to make the magnetic layer adhere strongly to the support.
For example, it is known that a compound having a functional group that is cured by radiation such as an electron beam, that is, a radiation curing compound, is used to form an undercoat layer.
For example, it has been proposed that a magnetic recording medium is prepared by forming an undercoat layer using a difunctional aliphatic compound as the radiation curing compound (ref. JP-A-60-133529, JP-A-60-133530, and JP-A-5-57647 (JP-A denotes a Japanese unexamined patent application publication)). These aliphatic compounds give a cured coating having a glass transition temperature of at most on the order of 40° C., and there is the problem that cohesive failure might occur during a coating step after the undercoat layer is applied. Furthermore, since such a smooth magnetic layer increases the contact surface between the surface of the magnetic layer and a head, the head is easily contaminated during repetitive sliding.
The contamination of the magnetic head can cause degradation in the electromagnetic conversion characteristics. In particular, high recording density equipment has an increased magnetic head rotational speed; in the case of a digital video tape recorder, the magnetic head rotational speed is 9,600 rpm, which is markedly higher than the 1,800 rpm for a consumer analogue video tape recorder and the 5,000 rpm for a business analogue video tape recorder, and the speed at which the magnetic recording medium and the magnetic head slide against each other is correspondingly high. Moreover, the magnetic head used therein is a small-sized type such as a thin film head, and there is a desire for an improvement in the magnetic head contamination caused by a component of the magnetic recording medium.
As a method for improving the above-mentioned problems, there is a method in which the hardness of the magnetic layer is improved by use of a hard binder. For example, a magnetic recording medium employing as the binder a polyurethane having a glass transition temperature of 80° C. or higher (ref. JP-A-2001-131258), and a magnetic recording medium employing as the binder a polyurethane resin containing a dimer diol as a diol component (ref. JP-A-11-96539) have been proposed. Furthermore, a magnetic recording medium employing as the binder of the magnetic layer an aromatic polyester having a glass transition temperature of 80° C. or higher (ref. JP-A-2001-6146), a magnetic recording medium employing a polyvinyl acetal resin having a glass transition temperature of 80° C. or higher (ref. JP-A-2001-134920), and a magnetic recording medium employing a binder having a glass transition temperature of 100° C. or higher (ref. JP-A-2002-25035) have been proposed.
Such use of a resin having a high glass transition temperature (Tg) as the binder enables the coating strength, in particular at high temperature, to be increased and good transport durability to be obtained. However, increasing a cyclic structure or a urethane bond component in order to increase the Tg results in a decrease in the solvent solubility, sufficient dispersibility thus cannot be obtained, and the coating smoothness is thereby degraded, which is a problem.